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Last edited 01 Sep 2020
Bored piles, also known as replacement piles, are a commonly-used form of building foundation that provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.
Bored piles are piles where the removal of spoil forms a hole for a reinforced concrete pile which is poured in situ. The spoil is replaced by the pile, hence 'replacement' piles as opposed to displacement piles where soil is forced away by driving or screwing the pile.
Bored piles are used primarily in cohesive subsoils for the formation of friction piles and when forming pile foundations close to existing buildings. They are popular in urban areas as there is minimal vibration, where headroom is limited, where there is no risk of heave, and if there is a need to vary the length of the piles.
In unstable soil strata, the use of bentonite fluid assists in stabilising the bore especially in large diameter deeper piles and allows the insertion of heavily reinforcing steel cages. This is known as flush boring (see more below).
Large diameter piles can have their bearing capacity increased by under-reaming the shaft at the base. This is achieved by an expanding cutting tool which cuts a conical-shaped base up to three times the diameter of the main shaft.
Where piling is undertaken in stable soils it may be possible to bore and place the concrete without first lining the hole. However, precautions need to be taken by lining the first metre of the hole to prevent surface spoil from falling into the hole.
The borehole can be formed by the percussion method which involves a heavy cutting tool on a small tripod which is dropped from its raised position using a winch to cut out a cylinder of earth. The operation is repeated until the hole has been sunk to the required depth. As the cutting proceeds, a thin sectional lining is inserted into the hole to prevent its collapse. Alternatively, the borehole can be formed by the rotary method in which the rotary drill operates within the casing or lining.
It is common for the borehole to be supported by a screw-jointed steel lining which is removed either during or after the concrete has been placed. The tube can be winched or jacked out of the ground.
Flush boring uses a fluid such as bentonite to flush material from the hole, which has been loosened by drilling. The fluid can be poured from above (reverse flushing) or pumped through the drilling rod (direct flushing). The bored hole is lined with a temporary steel casing to prevent collapse of loose surface soil. As the boring proceeds, bentonite is continuously fed into the hole.
At the required depth, reinforcement is lowered through the bentonite and concrete poured. The bentonite is displaced by the concrete and pumped back up out of the hole. As the concrete reaches the hole’s upper level, the temporary casing is withdrawn.
 Related articles on Designing Buildings Wiki
- Continuous flight auger piles.
- Driven piles.
- End-bearing piles.
- Friction piles.
- Geothermal pile foundations.
- Ground anchor.
- Moorfields building sets UK pile-loading record.
- Pad foundation.
- Pile cap.
- Pile foundations.
- Pile integrity test.
- Piling equipment.
- Screw piles.
- Secant pile wall.
- Sheet piles.
- Socket piles.
- Tension piles.
- Testing pile foundations.
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